1. Field of the Invention
This invention relates to a nickel, chromium, iron welding alloy, articles made therefrom for use in producing weldments, and weldments and methods for producing these weldments.
2. Brief Description of the Prior Art
In various welding applications, including equipment used in nuclear power generation, weldments are required that provide resistance to various cracking phenomenon. This includes not only stress corrosion cracking but hot cracking, cold cracking, and root cracking as well.
Commercial and military nuclear power generation have only existed within the second half of the 20.sup.th century. During this time, the industry has replaced the first generation of NiCrFe alloys having 14 to 15 percent chromium with alloys having higher chromium contents on the order of 30 percent. This change was predicated on the discovery that stress corrosion cracking in nuclear pure water could be avoided with alloys of this type that contained chromium in this amount. These alloys have been in use for about 20 to 25 years.
The specific application for nuclear power generation equipment that requires the majority of welding and welded products within the nuclear power plant is the fabrication of the nuclear steam generator. This equipment is essentially a large tube and shell heat exchanger that generates steam from secondary water from primary nuclear reactor coolant. The key component of this steam generator is the tubesheet. It is sometimes 15 to 20 feet in diameter and well over a foot thick and is usually forged from a high strength low alloy steel that must be weld overlaid with a NiCrFe alloy that has good fabric ability and is resistant to stress corrosion cracking in nuclear pure water. Due to the size of the tubesheet, the weld deposit sustains substantial residual stress during overlay. Furthermore, the weld metal overlay must be capable of being rewelded after being drilled to provide openings therein to receive thousands of small steam generator tubes. These tubes must be seal-welded to the overlay weld deposit to make helium-leak-tight welds. These welds must be of extraordinary high quality and must provide 30 to 50 year life with high predictability. In addition, in both the overlay weld deposit and the welded steam generator tubes, excellent crack resistance must be provided. This requirement, with respect to resistance to hot cracking, also termed "solidification cracking," and stress corrosion cracking has been met by most of the existing 30% chromium weldments.
In addition to hot cracking resistance and stress corrosion cracking resistance, the tube-to-tubesheet welds require root cracking resistance. The tube-to-tubesheet weld is made by melting the tube end together with a ring of the weld overlay material surrounding the tube (with or without the use of additional filler metal) to thereby seal the space between the tube wall and the opening in the tubesheet. There is a tendency for these welds to crack at the intersection of the weld at the joiner of the tube to the tubesheet. This type of cracking is referred to as "root cracking" because it occurs at the root of the weld. The existing 30% chromium welding alloys are not resistant to root cracking.
A third type of cracking that may be encountered is cold cracking, also known as "ductility dip cracking." This cracking only occurs in the solidified state after weld solidification has been completed. After solidification occurs, shrinkage stresses begin to develop as a result of the reduction in volume of the welding alloy at lower temperature. At the same time, once solidification is complete, ductility recovery occurs rapidly for a few hundred degrees, followed by a sharp temporary loss in ductility, and again followed by a more gradual continuous recovery of ductility until ambient temperature is reached. If the residual stress of cool-down is sufficiently large when the alloy exhibits this sharp ductility loss, solid state cracking may occur. This results from portions of the microstructure not having sufficient strength or ductility to resist the stress at the prevailing temperature. The commercially available 30% chromium welding alloys presently available are not sufficiently resistant to cold cracking.